Ancient supernovae may have accelerated evolution on Earth

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Earlier this year, astronomers produced evidence that the Earth was recently subjected to two “nearby” stellar explosions, supernovae that occurred about 300 light years away and roughly two and seven million years ago, respectively. Now, new evidence shows that those previously discovered supernovae would have had a profound effect on Earth, and potentially on the development of the life it held. Given that these were relatively recent events, they are absolutely proximate enough to have influenced the development of primate species that would go on to become mankind — but it could have influenced the evolution of all other species on Earth, as well.

The paper, appearing in Astrophysical Journal Letters, is based on computer modelling of the atmospheric and surface effects of the radiation sent to Earth from these supernova events. The expectation was that such distant explosions wouldn’t have a significant effect on the surface, but modelling revealed a significant uptick in surface radiation exposure. The effect would have been so strong that in the early days the night sky would probably have been lit up with the bright blue glow of radiation, disturbing animal sleep patterns.

We are constantly being bombarded by cosmic rays, but the Earth’s magnetic field keeps us mostly safe.

In particular, they found that there should be a major increase in the number of high-energy muons falling on the Earth, giving life on the surface potentially as much as 20 times the normal exposure. These quantum particles are created mostly when heavier particles like high-energy protons impact molecules in the upper atmosphere and destroy them, throwing off all sorts of short-lived pieces of debris. Since they’re generated in the upper atmosphere, even terminal particles like muons can live long enough to reach the surface.

Muons only partially interact with conventional matter, and the vast majority simply pass through us (and most of the Earth itself). But we’re still hit with so many that they account for about 1/6th of the average radiation dose — meaning that the muon increase alone could have raised the overall radiation level near the surface by as much as a factor of three. Every land animal on Earth, along with every marine animal living in shallow water, was subjected to about a CT scan’s worth of radiation every year.

Cosmic muons are generated in the upper atmosphere, then shower down on the Earth.

Played out over the billions of individual animals that would have been affected, this could have significantly affected the path of evolution. Increasing the average radiation dose would have increased the rate of new mutations, which could have in turn affected the rate of evolution. There may have been a period of the Earth’s ancient history in which animals were both subject to a higher-than-normal level of cancers, and a higher-than-normal level of long-term species change.

The latter process wouldn’t had had time to get all that far, what with the increased radiation lasting at most a few dozen generations. However, the supernova radiation may have had an equally profound effect on life, in a negative direction: mass extinction. Paradoxically, it seems that the influx of new stellar radiation may actually have contributed to rapidly cooling the planet by rapidly ionizing the upper atmosphere.

We can find a roughly corresponding extinction event in the fossil record, starting about 2.6 million years ago. The researchers describe the hypothesis as “controversial,” but they do float the possibility that cosmic rays from these two supernovae could have been a contributing factor to that period of extinction. Rather than directly killing them off with radiation poisoning, the idea is that the radiation rapidly changed the climate, and the world left a large fraction of the world’s biodiversity behind.

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